International Journal of Engineering Trends and Technology (IJETT) – Volume 24 Number 4- June 2015
Finite Element Analysis of Underground Water Tank with different
Safe Bearing Values of Soil
Punith C #1,Usha *2 , Jayalakshmi Raju *3, M. Adil Dar *4 .
#1
2
Lecturer,Civil Dept., SEACET,Karnataka, India
Lecturer, Civil Dept.,Brindavan College of Engg, Karnataka, India
3
P.G. Scholar ,SDDIET,Kurukshetra University, India
4
P.G. Scholar ,SDDIET,Kurukshetra University, India
Abstract— Underground liquid storage tank as part of
environmental engineering facilities and primarily used for water
and Sewage treatment plants and other industrial wastes. Normally,
they are constructed of reinforced concrete in the form of
rectangular or circular configurations. The behavior of liquid
storage underground tank during earthquakes is more important
than the economic values of the tanks and their contents. A good
understanding of the seismic behavior of these structures is
necessary in order to meet safety objectives while containing
construction and maintenance costs. In this paper Underground
water tank with different safe bearing values of soil is used to
analyze the typical behavior caused by seismic load. The finite
element method (FEM) is selected as the examination method for the
underground water tank. The most challenging part of this
dissertation is seismic calibration stage which is executed by
SAP2000 FEM package.
Keywords- underground water tank, Finite element modeling, earth
quake loads, SAP 2000.
I. INTRODUCTION
Storage reservoirs and underground water tank are used to
store water, liquid petroleum, petroleum products and similar
liquids. The force analysis of the reservoirs and tanks is about
the same irrespective of the chemical nature of the product.
All tanks are designed as crack free structures to eliminate any
leakage. Water or raw petroleum retaining slab and walls can
be of reinforced concrete with adequate cover to the
reinforcement. Industrial waste can also collected and
processed in concrete tanks with few exceptions. The
petroleum product such as petrol, diesel oil, etc. Are likely to
leak through the concrete walls, therefore such tanks need
special membrane to prevent leakage. Reservoir is a common
term applied to liquid storage structure and it can be below or
above the ground level. Reservoirs below the ground level are
normally built to store large quantities of water whereas those
of overhead type are built for direct distribution by gravity
flow and are usually of smaller capacity.
Fig 1: Detail of an RCC water tank
II. OBJECTIVES
To understand governing loads and carry out
literature review related to underground water tank.
Development of finite element model of underground
water tank using Structural Analysis Package (SAP
2000) software.
To study the behavior of underground water tank
under different safe bearing values of soil using
analytical methods.
To know about the design philosophy for the safe
and economical design of water tank.
To study the displacement and deformation pattern of
underground water tank and are compared for
different safe bearing values of soil.
To study the base moments, side wall moments and
settlement of underground water tank structure by
considering dynamic type of loading when the tank is
empty and full water level conditions.
III. DESIGN REQUIREMENTS
In water retaining structure a dense impermeable concrete is
required therefore, proportion of fine course aggregates to
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International Journal of Engineering Trends and Technology (IJETT) – Volume 24 Number 4- June 2015
cement should be such as to give high quality concrete.
Concrete mix weaker than m20 is not used. The minimum
quantity of cement in the concrete mix shall not be less than
30KN/m3 .The design of the concrete mix shall be such that
the resultant concrete is sufficiently impervious. Efficient
compaction preferably by vibration is essential. The
permeability of the thoroughly compacted concrete is
dependent water cement ratio. Increase in water-cement ratio
increases permeability, while concrete with lower water
cement ratio is difficult to compact. Other causes of leakage in
concrete are defects such as segregation and honey combing.
All joints should be water tight as these are potential sources
Fig 4. Underwater tank subjected to internal water pressure.
of water leakage. Design of liquid retaining structure is
different from ordinary R.C.C structures as it requires that
concrete should not crack and hence the tensile stresses in
concrete should be within permissible limits. A reinforced
concrete member of liquid retaining structure is designed on
the usual principles ignoring tensile resistance of concrete in
bending. Additionally it should be ensured that tensile stress
on the liquid retaining face of the equivalent concrete section
does not exceed the permissible tensile strength of concrete.
The co-efficient of expansion due to temperature change is 11
x 10-6 /oC and co-efficient of shrinkage may be taken as 450
x 10-6 for initial shrinkage and 200 x 10-6 for drying
shrinkage.
Fig 4
Underwater tank subjected to sub lateral soil pressure.
B) Bearing capacities of different rocks
Sl. no
1
2
3
Fig 2. Finite element analysis of underground water tank.
4
A) Finite element analysis of underground water tank.
1620
880
440
description
Gravel, sand and gravel, compact and
offering resistance to
penetration when excavated by tools
Coarse sand, compact and dry
Medium sand, compact and dry
Fine sand, silt (dry lumps easily
pulverized by fingers)
SBC
440
5
Loose gravel or sand gravel mixture,
Loose coarse to medium
sand, dry
245
6
Fine sand, loose and dry
100
2
3
4
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Sbc(kpa)
3240
C) Presumptive safe bearing values for different types of
Sandy soil
Sl no
1
Fig 3. FEM analysis of underground water tank subjected to dry soil
pressure
description
Rocks (hard) without laminations and
defects. For e.g. granite
trap & diorite
Laminated Rocks. For e.g. Sand stone and
Lime stone in sound
condition
Residual deposits of shattered and broken
bed rocks and hard
shale cemented material
Soft rock
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440
245
150
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International Journal of Engineering Trends and Technology (IJETT) – Volume 24 Number 4- June 2015
D) Presumptive safe bearing values for different types of
Cohesive soil
Sl
no
description
SBC
1
2
Soft shale, hard or stiff clay in deep bed, dry
Medium clay readily indented with a thumb
nail
440
245
3
Moist clay and sand clay mixture which can
be indented with strong thumb pressure
150
4
Soft clay indented with moderate thumb
pressure
100
Fig 6 moment co-efficients
5
Very soft clay which can be penetrated
several centimeters with
the thumb
50
6
Black cotton soil or other shrinkable or
expansive clay in dry
condition (50 % saturation)
130160
Moment Co-efficient and individual wall panels, top and
bottom hinged.
Vertical walls fixed.
Hence appendix 01 is created which is based on IS 456-2000
and the design of top
Slab has been carried out.
Assumptions for design of top slab:
Loads:
DEAD LOAD = 2KPA
IV. ANALYSIS AND DESIGN
LIVE LOAD = 5KPA
DESIGN CASES:
Case 1: Tank is full and
Case 2: Tank is empty.
The above cases are extreme cases and hence the underground
tank design has been carried
out considering these two cases.
FOLLOWING
MODEL:
INPUT DETAILS ARE CONSIDERED IN THE
1)
K SPRING CONSTANT = 250 / 0.005 = 50,000
2)
UNIT WEIGHT OF SOIL = 18 KN / M3
DESIGN CALCULATIONS:
3)
KA = 0.33
Case 1: Tank is full .
a. Maximum soil pressure = 18000 x 4.875 x (1-sin30/
1+sin30)
Ps = 32951.5 N/m2
Ps = 32.95 KN/m2
b. Maximum water pressure = 9810 x 4.875
Pw = 47823.8 N/mm2
Net pressure at the bottom of wall = Pw - Ps
Pnet = 14872.25 Nm2
These pressure values have been incorporated in the finite
element software.
Simultaneously these values are utilized for the further
computation of forces.
4)
UNIT WEIGHT OF WATER = 10 KN/M3
5)
SURCHARGE PRESSURE = 10 KN/ M2
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6) DEPTH OF OVERBURDEN = 3.15M
By considering the above loads and the geometry, the
following mathematical model
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FEM
International Journal of Engineering Trends and Technology (IJETT) – Volume 24 Number 4- June 2015
has been created.
Fig 6 3D complete model
Fig 8 3D model resultant BMD M11 direction minimum
Combination : ENEVOLOP { case 1, case 2 }
Case 2 : Tank is full
1.5 x (Self weight + lateral submerged soil pressure +
surcharge+ live load + floor finish)
Fig 7. 3D model hiding top slab to highlight internal walls
Case 1 : Tank is full
1.5 x (Self weight + internal water pressure + lateral dry soil
pressure
+ Surcharge + live load + floor finish)
Fig 9 3D model resultant BMD M11 direction maximum
Note: Since it is envelop minimum and maximum forces are produced.
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V. CONCLUSIONS
TIME PERIOD AND FREQUENCY
1) It is well known that natural time period goes on decreasing
with increasing in frequency.
2) Time period is high when the underground water tank is
full water level compared to empty. If the underground water
tank is full of water obviously it takes more time to complete
one cycle of free vibration than the empty tank.
3) Frequency is less when the tank is full compared to empty.
If the tank is full obviously it takes less rotation per second.
DEFORMED SHAPE DUE TO DIFFERENT LOADINGS
The deformed shape are extracted for safe bearing value
500KN/m2
1) The deformation is negligible in all loading conditions.
2) The maximum deformation due to full water load is only
364*E-3m.
3) Due to uniform distribution of water, and external loading
at top, the deformation is more at the center of the side walls
of tank.
4) Since the vehicular load (live load) at the top portion, hence
the maximum deformation is at top portion of tank.
JOINT DISPLACEMENT AND JOINT ACCELERATION
1) Displacement value is decreased with increased in safe
bearing values, in all conditions.
2) Due to water hammering action displacement is high in full
water level condition compared to empty condition in both X
and Y direction, due to seismic vibrations.
3) Acceleration value is decreased with increased in safe
bearing values, in all conditions.
4) Acceleration is little bit high in full water condition due to
water hammering action by earthquake compared to empty
condition in both X and Y direction.
BASE MOMENT
1) Base moment value is very high in safe bearing value
150KN/m2 compared to safe bearing value 500KN/m2 , in all
conditions.
2) Due to high lateral force and water force, the base moment
is high in full water level condition compared to empty
condition in all considered safe bearing values.
SETTLEMENT DUE TO SEISMIC LOADS
1) Settlement value is high in safe bearing value 150KN/m2
compared to safe bearing value 500KN/m2 , in all conditions.
2) Due to water load, the settlement is high in full water level
condition compared to empty condition in all considered safe
bearing values.
SIDE WALL MOMENTS
1) The side wall moment in all considered safe bearing values
and in all conditions is almost linear . But if we compare
among those water level, side wall moment is little bit varying
in full water condition compared to empty condition.
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2) During earthquake, due to water hammering action the
displacement is little bit high in safe bearing value 150KN/m2
compared to safe bearing value 500KN/m2 .
3) The side wall moment is decreasing with increasing safe
bearing values.
With this, finally we concluded that, for every load case the
soil with safe bearing value 150KN/m2 has higher value.
Hence the underground water tank in soil with safe bearing
value 150KN/m2 will get affect more than the underground
water tank in soil with safe bearing value 500KN/m2 during
earthquake.
Therefore for heavy and important structures such as
underground liquid storage tanks, dams, bridges etc., the soil
with higher safe bearing values are preferred. Since joints are
the weakest positions in the underground water tank, hence
providing the rubber dampers at the joint segment is one of
the methods in earthquake resistant design. This method can
be use practically by conducting some more investigations and
experiments to make underground water tank earthquake
resistant.
VI. SCOPE OF FUTURE WORK
1. The analyses can be extended by considering when tank is
half water level condition. In this way, the behavior of
different safe bearing values of soil are observed more
sensitively.
2. Furthermore, the analysis can be carried out by considering
circular shape of underground water tank.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 24 Number 4- June 2015
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CODES/STANDERDS
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[15] Indian Standard IS 1893 (Part 1): 2002 “Criteria for Earthquake
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